In recent years, marked advances in the miniaturization of mobile communication terminals, such as mobile phones and Wireless LAN (Local Area Network) routers, has been achieved due to the miniaturization of the various components incorporated therein. One of the most important components incorporated in a communication terminal is the filter.
In particular, bandpass filters are often used in communication applications for blocking or filtering signals with frequencies outside a certain passband. In such applications, bandpass filters preferably exhibit low insertion loss and steep roll-off attenuation at passband edges (i.e., the upper and lower frequencies of the range that are not highly attenuated by the filter). Out-band rejection or attenuation is an important parameter for a bandpass filter. It measures the filter's capability of discriminating in-band and out-band signals. The bigger the out-band rejection and the wider the rejected bandwidth, normally the better the filter. Also, the steeper the rolloff frequency edge between pass-band and out-band, the better the filter. To achieve rapid rolloff, more resonant circuits or more filter sections are typically required. This generates more transmission zeros at out-bands, leading to higher order of out-band attenuation. Unfortunately, using more sections and resonant circuits increases filter dimensions and a filter's insertion loss in the pass-band. This is not helpful for the miniature requirement in modern wireless communication systems.
For example, conventionally, low-loss high quality factor microwave resonator circuits are used to achieve steep roll-off attenuation. Microwave resonator circuits typically utilize quarter-wavelength or half-wavelength transmission line structures in order to realize low losses at microwave frequency. For lower gigahertz wireless applications, quarter-wave or half-wavelength structures demand large component size in order to accommodate the transmission line structures. Such large components are unsatisfactory for use in smaller electronic devices.